Grain oriented electrical steel sheets are mainly used as iron core materials for transformers, electrical equipment motors, and so on. From the point of view of energy saving, grain oriented electrical steel sheets having excellent magnetization characteristics, in particular, grain oriented electrical steel sheets that provide low iron loss are desired.
In the manufacturing of such grain oriented electrical steel sheets, a steel slab including elements for forming an inhibitor, for example, MnS, MnSe or AlN, that is necessary to induce secondary recrystallization (for example, the elements in the case of MnS are Mn and S) is hot rolled to form a steel sheet. The steel sheet is annealed as required, subsequently cold rolled one time, or two or more times with intermediate annealing after each pass to attain a final sheet thickness, then decarburized annealed, coated with an annealing separator such as MgO, and subjected to final finishing annealing.
In most cases, on the surface of a grain oriented electrical steel sheet manufactured by the above process, a ceramic insulating film called forsterite (Mg2SiO4) (also written as forsterite film) is formed. Usually, a tension coating is formed on top of this film to attain enhanced insulating properties and to give a tension to the steel sheet.
When such grain oriented electrical steel sheets are used as layered iron cores, the forsterite film and the tension coating film provide electrical insulation between layers of the steel sheet and contribute effectively to reducing the eddy current. One type of such layered iron cores is stacked iron cores in which a plurality of flat sheets that have been shorn or punched are piled on top of one another (see, for example, Patent Literature 1). Another type is wound iron cores that include a series of circles produced by coiling of a flat sheet (see, for example, Patent Literature 2). If the adhesion properties of a forsterite film are low, the forsterite film is easily exfoliated from the steel sheet surface at punched ends of a stacked iron core or at bent portions in a wound iron core. In the presence of exfoliation in the forsterite film, the electrical insulation between layers of the steel sheet is lowered by tightening during the fabrication of the iron core. Poor electrical insulation causes a local generation of heat when the layered iron core is used in a transformer or a motor, giving rise to a decrease in the performance of electrical equipment and, even worse, a risk of accidents such as dielectric breakdown. Further, poor adhesion between the steel sheet surface and the forsterite film facilitates the exfoliation of the forsterite film not only during the deformation of the steel sheet but also by the collision of foreign objects. This exfoliation usually occurs at the interface between the steel sheet and the forsterite film. The exfoliation in forsterite films possibly leads to corrosion of the steel sheets during the period between the manufacturing of the steel sheets and the fabrication of iron cores. The corrosion of steel sheets decreases the commercial value of the grain oriented electrical steel sheets.
To achieve a further reduction in the iron loss of grain oriented electrical steel sheets, a method for magnetic domain refining has been presented and been in practical use. In this method, magnetic domains are subdivided by forming defects such as grooves on the surface of a final cold-rolled steel sheet, or by introducing local strains into the steel sheet surface by the irradiation of an electron beam (electron ray) or a laser beam after secondary recrystallization annealing. Of these magnetic domain refining techniques, the introduction of local strains takes place after the formation of a forsterite film. If the forsterite film has low adhesion properties, the forsterite film and further the tension coating are exfoliated together from the steel sheet depending on the conditions under which an electron beam or a laser beam is irradiated. The exfoliation of these films from the steel sheet results in a significant decrease in the insulating properties and corrosion resistance of the grain oriented electrical steel sheet. A countermeasure to this problem is to apply another coating in order to cover portions exposed by the exfoliation. However, this approach not only increases the manufacturing costs of grain oriented electrical steel sheets, but also causes the tension coating to undergo baking, which relaxes the local strains and significantly decreases the effect of magnetic domain refining.
It is of great importance to increase the adhesion properties between a steel sheet and a forsterite film and to minimize the occurrence of exfoliation. In particular, high-grade grain oriented electrical steel sheets which are processed by magnetic domain refining treatment are required to have strong resistance to exfoliation, that is, to have high adhesion between a forsterite film and the steel sheet surface.
A forsterite film also serves to enhance magnetic characteristics by giving a tensile stress to the surface of a steel sheet. Thus, the film is demanded to have excellent film adhesion and to have a high performance in giving a tensile stress to the steel sheet surface. Here, the term “film adhesion” means adhesion properties between the forsterite film and the surface of the steel sheet.